The present invention relates to a high strength polyethylene naphthalate fiber useful for the reinforcement of rubber products such as tires.
A polyethylene-2,6-naphthalate has a higher glass transition temperature, crystallization temperature, melting temperature and melting viscosity, as compared to a comparable polyethylene terephthalate, and thus, it is generally spun at a temperature higher than the spinning temperature (310 to 320xc2x0 C.) of a conventional polyethylene terephthalate. However,at a high spinning temperature, heat decomposition takes place to some extent, resulting in a reduction of the intrinsic viscosity, which makes it difficult to prepare a polyethylene-2,6-naphthalate yarn having the original high strength (see Japanese Patent Publication Nos. 72-35318, 73-64222 and 75-16739).
Japanese Patent No. 2945130 describes a method of making polyethylene naphthalate fibers having high tenacity and high modulus by way of controlling the spinning speed and spinning draft ratio and changing the drawing temperature stepwise, instead of raising the spinning temperature. According to this method, however, uniform spinning cannot be accomplished and the drawing process itself is difficult to control because of a high first-step drawing temperature of more than 150xc2x0 C.
The present inventors have endeavored to develop an improved polyethylene-2,6-naphthalate fiber and have discovered that a high strength fiber can be prepared by way of controlling the properties of polyethylene-2,6-naphthalate polymer itself and the drawing conditions while maintaining a relatively low spinning temperature.
It is, therefore, an object of the present invention to provide a high strength polyethylene naphthalate fiber having high tenacity and improved dimensional stability, suitable for use as a reinforcing cord.
In accordance with one aspect of the present invention, there is provided a polyethylene naphthalate fiber prepared by a method comprising the steps of:
(A) melt-spinning a solid-phase polymerized polyethylene-2,6-naphthalate chip, composed of 95 mol % or more of ethylene-2,6-naphthalate units as the recurring unit, and having a manganese content of 30 to 70 ppm, an antimony content of 180 to 300 ppm and an intrinsic viscosity of 0.90 to 1.00, through a spinneret at a spinning draft ratio ranging from 400 to 900 to form a molten spun yarn,
(B) rapid-cooling and solidifying the spun yarn by passing it through a solidification zone,
(C) withdrawing the solidified yarn at such a rate that an undrawn yarn having a birefringence of 0.005 to 0.025 is formed, and
(D) subjecting the undrawn yarn to a multi-stage-drawing process to a total draw ratio of 6.5 or more to obtain a fiber, wherein the temperature of the second drawing stage is maintained in the range of 150 to 180xc2x0 C.; said fiber having (1) an intrinsic viscosity of 0.70 to 0.77, (2) a tenacity of at least 9.5 g/d, (3) an elongation of at least 9.5%, (4) a birefringence of at least 0.45, (5) a density of 1.355 to 1.368, (6) a melting point of 270 to 275xc2x0 C., and (7) a shrinkage of 2 to 3%.